PURPOSE: To suppress a newly necessary factor to a minimum limit and to reduce its cost with space conservation by separating between a driving circuit of a motor and the motor by a switch control for a short time, and measuring a voltage due to a counterelectromotive force as information regarding a rotating number of the motor.

CONSTITUTION: The set signal of the rotating number of a motor to be output from a rotating number setter 1 is input to a multiplexer 12. The multiplexer 12 is also connected to the lead points 21, 22, in addition to the setter 11. A signal to be input from the setter 1 or the points 21, 22 is supplied to an A/D converter 13 by a control signal of a microcomputer 14 in a suitable time zone, A/D-converted, and then read by the microcomputer 14. The signals obtained from the points 21, 22 are as motor rotating number information regarding the rotating number of the motor 7, and the microcomputer 14 calculates an error of the rotating number of the motor 7 by the set value and the speed information.